Someone, on checking out my github page, recently commented that I’m more of a breadth than a depth developer. I’m not sure that this is entirely accurate, though I didn’t take offence, but it is true that many of my personal projects are small one-off toys. This post is a tour of some of them.
I didn’t study Computer Science at university1 and I sometimes feel like I missed out on some key programming projects like writing your own operating systems, compilers, and path tracers. I’ve been steadily remedying this and dmanix is my attempt at an operating system.
It only runs on 386 and has a scheduler, ramdisk, memory allocation, etc. The next step will be writing an ELF parser and compiling programs for it.
The only thing that separates it from other *nix OSes is that it’s written in more C++ than C.
One of the interview questions I ask on occasion2 is to implement an LRU cache in C or C++. The idea behind an LRU cache is that you have a fixed capacity cache in which you want to store data such that only the most recently used things are in it. In this case, I’m storing key/value pairs.
The specific reason why this is an interesting implementation is that it uses only operations that are average-case O(1). Ie, it should scale without significant performance reduction. This is managed by using both a list and an unordered_map; the unordered_map to store the key-value pairs for fast lookup, and the list to track how recently items are accessed.
There’s a well-known class of puzzles in which one is asked to fill a container using two other odd-sized containers. For example, fill a 5l container using a 3l and 4l container. I don’t enjoy these puzzles so I decided to write code to solve them for me.
It uses a depth-first graph search algorithm to explore the solution space and then prints out the steps needed to solve the problem. There’s one issue which I should fix at some point: It might be better to use a breadth-first search as the solution space is relatively small and this would return the shortest solution.
One of the things I’ve been doing recently is learning Go3. I felt that I needed to get to grips with the scalability and use for a web service, so I built a highly concurrent queue server and client.
It uses JSON over HTTP as a wire protocol and supports many, many concurrent clients thanks to Go’s built-in support for concurrent HTTP serving. The server is using lock-free queues to maximize concurrency.
Evernote have a series of coding challenges on their careers website. I enjoy a challenge so I tackled the problems. No-one ever contacted me so I don’t know if I did well, but my code passes all of the test cases and is relatively fast. All the other solutions posted around the web are in Python (that I found, anyway) so I thought having C++ solutions would be a neat change.